So, first welcome to the forum. As far as I can see you are doing all the right things. Most chickens' eggs decrease both in number and quality over time and some of yours could qualify as middle aged now. There are products on the market which purport to improve shell strength. I have not tried the...

I have no experience of them myself but you might check out mumsnet.com as I note that in 2017 there was discussion on Arkus coops. By now, the owners should have a year's experience of how they perform and you could maybe wake up the thread to find out more.

No real answer for you but I had an OEG bantam with the same thing and it didn't seem to bother her any. I took her to the vet who removed some of it but it was stressful for her and as she was an old bird, I didn't want to continue. The bit that the vet removed grew back and blocked her nostril aga...

Personally I don't think you would notice any difference. If your chooks get a balanced diet (ie access to grass, bugs, weeds etc) then they'll do fine on your local stuff. If you feel strongly about GM then that is a different matter altogether.